In the case in which traffic information is to be provided to a navigation onboard machine for mounting a digital map data base, conventionally, a road is specified by a link number and a node such as an intersection present on the road is specified by a node number and a point on the road is transmitted in such a method as to represent a distance from the node in such a manner that a position on the digital map is accurately transmitted even if a transmitting side and a receiving side hold digital maps created by different sources.
However, the node number and the link number which are defined in a road network are to be changed to new numbers corresponding to the new provision or alteration of a road. Moreover, the digital map data of each company to be a creation source are to he also updated correspondingly. Therefore, the methods using the node number and the link number require a large social cost for maintenance.
In order to improve such a respect, the inventors of the invention have proposed the following digital map position information transmitting method in JP-A-11-214068 and JP-A-11-242166.
In such a method, when the information providing side is to transmit the position of a road in which an event such as a traffic jam or a traffic accident is caused, “road shape data” comprising the coordinate string of a node and an interpolation point (the vertex of a polygonal line approximating the curve of the road which will be referred to as a “node” including the interpolation point if there is no restriction in this specification) arranged on the road taking a shape in a road section having a predetermined length which includes the event position and “event position data” indicative of the event position based on a relative position in the road section represented by the road shape data are transmitted to the receiving side, the side receiving these information carries out map matching by using the road shape data to specify the road section on a self-digital map, thereby specifying an event generation position in the road section by using the event position data.
FIG. 43 illustrates the “road shape data” and FIG. 44 illustrates the “event position data”.
In a method of transmitting the position information of the digital map by using the “road shape data” and the “event position data”, however, there is a problem in that the data volume of the road shape data for specifying the shape of a road is increased and the amount of data transmission is thereby increased.
As a method of reducing the data volume of the road shape data, the inventors of the invention has proposed a method of approximating the shape of a road by a spline function in JP-A-2001-12127. In order to fix the position information transmitting method, it is necessary to further promote a reduction in the data volume.
The invention solves such a problem and has an object to provide a position information transmitting method of transmitting position information of a digital map in a small data volume by utilizing a compression and coding technique, a coding method of reducing a data volume and a device for executing the methods.